This invention relates to integrated circuits, and more particularly to circuitry on integrated circuits that facilitates the performance of certain kinds of arithmetic operations within the integrated circuit.
Baeckler et al. U.S. patent application Ser. No. 10/718,968, filed Nov. 21, 2003 (hereby incorporated by reference herein in its entirety) shows examples of logic circuitry (e.g., on an integrated circuit) that can efficiently perform the addition of three multi-digit or multi-bit, variable, binary numbers. The Baeckler et al. reference shows doing this by separately forming a “redundant form” sum bit and a “redundant form” carry bit from the three bits in each “place” (bit position) of the numbers to be added. The redundant form sum bit is the binary sum of the three bits to be added in that bit position. Any carry from that operation is ignored. The redundant form carry bit is equal to the “majority” of the three bits to be added in that bit position (e.g., 0+0+0=0, 0+0+1=0, 0+1+1=1, and 1+1+1=1). Ripple carry adder circuitry is then used to combine the redundant form sum and carry bits to produce the final sum of the three inputs. In particular, each “stage” of the ripple carry adder receives (1) the redundant form sum bit for a respective one of the bit positions of the inputs, (2) the redundant form carry bit from the next less significant bit position of the inputs, and (3) a ripple carry bit output by the ripple carry adder stage for the next less significant bit position of the inputs. Each stage of the ripple carry adder produces (1) a final sum output bit equal to the sum of the inputs that this ripple carry adder stage receives, and (2) a ripple carry output bit equal to the carry (0 or 1) from the last-mentioned sum.
Some applications of circuitry of the type described above may require the addition of very long numbers (e.g., more than 90 binary digits (“bits”)). This requires use of a correspondingly long ripple carry adder. It can take a relatively long time for signal information to propagate through such a ripple circuit. This can increase the operating cycle time of the circuitry, which can be undesirable.